In a furnace of a fluidized bed boiler, chemical energy of a suitable fuel is converted to thermal energy by combusting it in a bed of inert material, which is arranged in the furnace and fluidized by air. In fluidized bed boilers, it is possible to bind a considerable portion of the sulfur released from the fuel by means of a sulfur-binding agent, usually limestone, being fed to the furnace. The calcium carbonate CaCO3 of limestone calcinates in the furnace to calcium oxide CaO, which forms with sulfur, calcium sulphate CaSO4 and calcium sulphite CaSO3. In order to achieve a good sulfur-binding level, an excess of limestone, compared to the amount of the sulfur in the fuel, must be introduced, due to which part of the calcium oxide is left over as being unreacted in the ash to be removed from the boiler, which again impedes the end storage of the ash.
In a fluidized bed boiler, heat energy is recovered both with heat surfaces arranged directly to the furnace and various heat exchange means arranged in a flue gas channel. In the parts of the flue gas channel, where the temperature of the flue gases and the temperature of the surfaces of the heat exchangers remain sufficiently high, it is possible to manufacture the heat exchangers of relatively inexpensive materials.
In modern thermal power plants with a high efficiency, heat energy from flue gases is efficiently recovered by cooling the flue gases to as low a temperature as possible. When flue gases are sufficiently cooled down, for example, to 90° C., water vapor in the flue gases may condense to droplets on the surfaces of the heat exchanger. Thereby, compounds in the flue gas, especially sulfur trioxide SO3 and sulfur dioxide SO2, can be solved to the water layer of the heat exchanger surface and form acids, such as sulfur acid H2SO4 and sulfurous acid HsSO3, which will corrode metal surfaces.
In general, corrosion has been attempted to be minimized by manufacturing the heat exchangers of a material resistant to corrosion as much as possible. Recently, especially when the flue gases contain aggressive compounds, the trend has been, however, to manufacture heat exchangers of non-corrosive materials, for example, of Teflon® or some other suitable plastic material. For example, U.S. Pat. No. 4,557,202 discloses some methods to utilize, in a thermal power boiler, corrosion-free heat exchangers manufactured of plastics, in a so-called condensing mode.
In heat exchangers containing plastic parts, the actual heat recovery tubes being in contact with the flue gases are usually vertical or horizontal plastic tubes, or tubes covered with plastics, which are connected with metal headers. The headers again are connected with recirculation piping for a heat exchange medium, most usually, water.
When a continuous flue gas flow hits the heat exchange surface of the heat exchanger, which is at a temperature lower than the acid and water dew point, it is possible that large amounts of a corrosive liquid, such as a water solution of sulfur acid H2SO4 and sulfurous acid H2SO3 condenses on the surfaces. Thereby, a corrosive liquid, a so-called condensing liquid, may flow downwards in the flue gas channel until it is removed through a discharge channel for liquid arranged in the channel. The condensing liquid collected from the flue gas channel has to be neutralized before it can be positioned to its final collection place. The neutralization is carried out usually in a special water treatment system, which causes additional operation and equipment costs.